[MITgcm-support] Behavior of Ptracers at Orlanski boundary

[Martin Losch]

Hi Yilang,

I am not sure about the combination of your orlanski and rbcs, maybe test if turning off rbcs will help.
Maybe you can use exp4 as a simple example. I recall that there, the ptracer leaving the domain it actually works quite well and I don’t see, why the orlanski BC should destroy that behavoir. 

Martin

> On 15. Apr 2019, at 08:10, Yilang Xu <yxu at whoi.edu> wrote:
> 
> Hi Martin and Jean-Michel, 
>  
> Thanks very much for your advice. I set up a test case and add " OBCS_u1_adv_Tr(1) = 1," in data.obcs, but the plot behaves the same as the case does without it. Generally, the problem is that the ptracer starts to accumulate to a large number after it reaches the Orlanski boundary. I am trying to use ptracer to mimic salt. They behave relatively in a similar way except for the cells near the boundary. I have attached a vertical slice of the ptracer for demonstration. 
>  
> My current implementation is the same as suggested in the old post (http://mailman.mitgcm.org/pipermail/mitgcm-support/2012-October/008012.html). Ptracer is resotred by RBCS on a small surface area, and " OBCS_u1_adv_Tr(1) = 1," is the only ptracer-related line in data.obcs (but this did not change any behavior). I am wondering what could be improved if I want to advect the ptracer out properly. Implementing Orlanski BC for ptracers could be a choice in the future. 
>  
> Thanks,
> Yilang
>  
>  
> On 4/12/19, 12:03, "MITgcm-support on behalf of Jean-Michel Campin" <mitgcm-support-bounces at mitgcm.org on behalf of jmc at mit.edu> wrote:
>  
>     Hi Yilang,
>     
>     Did you try, for each passive tracer "iTr":
>      OBCS_u1_adv_Tr(iTr) = 1,
>     This will force to use 1rst Order upwind advection-scheme at OB location 
>     but only in the case of an outflow. This could help the passive tracer to
>     leave the domain without accumulating.
>     An example can be found here:
>      verification/so_box_biogeo/input/data.obcs
>     
>     Cheers,
>     Jean-Michel
>     
>     On Fri, Apr 12, 2019 at 09:41:44AM +0200, Martin Losch wrote:
>     > Hi Yilang,
>     > 
>     > I think that your only way out is to change the boundary conditions for the passive tracers. You obviously read this old post. Now you know what I mean by “keep your fingers crossed”.
>     > 
>     > As a quick fix, I usually try to use some approximate v.Neumann BC, but that’s already implemented in the example given in the old post <http://mailman.mitgcm.org/pipermail/mitgcm-support/2012-October/008012.html>
>     > 
> > If that doesn’t help you’ll probably have to try to implement Orlanski BC’s for passive tracers …
>     > 
>     > Martin
>     > 
>     > > On 11. Apr 2019, at 21:51, Yilang Xu <yxu at whoi.edu> wrote:
>     > > 
>     > > Hi everyone, 
>     > >  
>     > > As MITgcm suggests, the implementation of Orlanski OBCS and Ptracers together is not recommended by the current version of the code. Following some previous discussions
>     > > (e.g., http://mailman.mitgcm.org/pipermail/mitgcm-support/2012-October/008012.html), I comment out the part that stops the model in obcs_calc.F and obcs_check.F. 
>     > >  
>     > > As a result, the model behaves well until the ptracer reaches the Orlanski boundary. I notice the occurrence of instability or aggregation of ptracer when it comes near boundary cells. 
>     > > I wonder if there is any solution to this problem. Appreciate your help. 
>     > >  
>     > > Thanks,
>     > > Yilang
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